Cavity backed slot antenna with rotatable loop feed



A. R. SISSON July 19, 1966 CAVITY BACKED SLOT ANTENNA WITH ROTATABLE LOOP FEED 2 Sheets-Sheet 1 Original Filed Aug. 5, 1961 IN VEN TOR.

A. R. SISSON ATTORNEY July 19, 1966 A. R. SISSON 3,262,119

CAVITY BACKED SLOT ANTENNA WITH ROTATABLE LOOP FEED Original Filed Aug. 5, 1961 2 Sheets-Sheet 2 C: 40 4/ 30 NORMAL OPERATION T 36 40 CAVITY WALL Y 36 CURRENT FLOW I CAVITY E FIELD CAVITY WALL CURRENT FLOW TEST OPERATION ia: a:

CAVITY 4 E FIELD RANGE INDICATOR SELF TEST MARKER RECEIVER ANTENNA TRANSMITTER RECEIVER SELF TEST BUTTON TRANSMITTER COUPLER ANTENNA INVENTOR.

A. R. SISSON ATTORNEY United States Patent 3,262,119 CAVITY BACKED SLOT ANTENNA WITH ROTATABLE LOOP FEED Austin R. Sisson, Canoga Park, Calif, assignor to The Bendix Qorporation, a corporation of Delaware Continuation of application Ser. No. 129,189, Aug. 3, 1961. This application July 3%), 1965, Ser. No. 477,088 12 Claims. (Ql. 343-703) This invention relates to high-frequency antennas and, more particularly, to slot-type aircraft radar antennas.

The present application is a continuation of my application Serial No. 129,180, filed August 3, 1961, now abandoned.

In the design of antennas for use in radar systems on fast flying aircraft, the merits of the flush-mounted slot antenna are well recognized. Ordinarily they employ a transmission line terminating at a ground plane which in actuality is a continuation of the aircraft skin. The continuous metallic surface made up of the aircraft skin and antenna body is interrupted by at least one substantially rectangular slot constituting the radiating portion of the antenna. In the past, a pair of slots have often been used to obtain a preferred directional pattern. For example, when such slot doublets are spaced at a half-wavelength spacing, a cosine radiation pattern is produced. Typically, slot antennas are fed by a waveguide with the Walls defining the slot, or by a coaxial conductor with the center conductor connected to a mid point of the longer side of the radiating slot or slots. One other type of feed for such slot antennas is through a probe positioned in a resonant cavity where the slot or slots are in an end wall of the cavity.

In certain radar applications it is essential that a provision be made for switching the antenna from radiating to nonradiating condition. An example of such a system is one intended [for a relatively broad band use employing several slot antennas, each tuned to a selected frequency or oriented in a selected way. Such a system requires one or more antenna switches to energize the proper antenna. An obvious way of accomplishing this effect is to employ a switch in the transmission line between the transmitter and an antenna. However, both coaxial and waveguide switches suffer from the defects of having less than 100% isolation between the antenna and the transmitter when in the open condition, and invariably offer an impedance mismatch with resultant reflections and loss of enengy. Moreover, both types of switch structures characteristically are complex mechanically as we'll, often resulting in short operating lives.

Still another type of radar system in which it is desirable to disable the antenna is one employing a self-testing feature. Such a feature employs a transmission line between the transmitting antenna and the receiver having a predetermined delay. In order to check the operation of the entire system, it is possible to switch from the transmitting antenna to the input connection to the delay line so that pulses generated by the transmitter reach the receiver at a known delay. It is essential in such a system that the antenna be disabled and no RF transmission permitted, since the receiver would otherwise detect both the energy coming through the delay line for test purposes and any reflected energy arriving at its own receiving antenna.

With this understanding of the requirements of the art for aircraft type slot antennas in mind, it is a general object of this invention to provide an efficient, small, slottype antenna for operation in the 4000-megacycle frequency range.

Another object of this invention is to provide such an antenna with a self-contained switching mechanism to allow the selective enablement and disablement of the antenna.

Still another object of this invention is to provide a slot-type antenna with an integral switch and including means for sampling energy reaching the antenna during periods of nonradiation.

One further object of this invention is to provide an effective means for feeding a dual slot of an antenna in phase with energy of equal magnitude in order to provide a preferred radiation pattern.

These objects are all achieved in accordance with this invention, one embodiment of which comprises an antenna body including a planar face and defining a cylindricalshaped cavity with one end wall of the cavity coextensive with the planar face. A pair of substantially rectangular slots in the front face of the antenna body constitute the radiating sunfaces of the antenna. An input loop for coupling electromagnetic energy'into the cavity is positioned in the center of the body entirely along the longitudinal axis of the cylindrical cavity and mounted for rotation about that :axis. A number of pillars, in this case four, extend into the cavity from the rear wall parallel to the cavity axis and at equal distances from the axis to provide capacitative loading. One of the pillars mounts a probe employed for detecting energy within the cavity under test conditions.

One feature of this invention involves the combination of a resonant cavity having a pair of energy-radiating slots in one wall of the cavity and means for introducing electromagnetic enengy into the cavity symmetrically with respect to the slots.

Another feature of the invention is the provision for rotating the energy-coupling means so that energy is introduced into the cavity selectively in two modes, one for efficient radiation by the slots and the other for relatively insignificant radiation by the slots.

Still another feature of the invention relates to the presence of symmetrically placed loading means within the chamber and including means for detecting energy in the chamber in the nonradiating mode.

These and other features of this invention may be clearly understood from the following detailed description and by reference to the drawing, in which:

FIGURE 1 is a diametric section through an antenna of this invention.

FIGURE 2 is a front elevational view of the antenna of FIGURE 1, with portions broken away for clarity.

FIGURES 3a and 3b are simplified elevational and sectional views, respectively, showing the coupling loop in energy-radiating orientation.

FIGURES 4a and 4b are simplified showings similar to FIGURES 3a and 3b with the coupling loop shown in nonradiating orientation.

FIGURE 5 is a simplified block diagram of a radar system employing this invention.

Now referring to FIGURE 1, an antenna 10 embodying this invention is made up of a unitary metallic body 11 including a flange portion 12 and a recess portion 13, the latter of which defines a cylindrical-shaped cavity 14 with the axis A of the cavity extending normal to the plane flange portion 12. The cavity 14 is closed by an apertured metallic plate 15 resting on a circular step 16 and covered by a circular dielectric disk 20 constituting the front face of the antenna 10 and forming a weather seal for the antenna. The disk 20 is transparent to radiated electromagnetic waves emanating from apertures in the plate 15.

The apertures in plate 15 are in the form of a pair of parallel-sided slots 21 and 22 positioned equidistant from the longitudinal axis A of the cavity. These slots 21 and 22 are best seen in FIGURE 2 as including straight side portions 23 and circular end-loading regions 24.

A coaxial conductor extends into the antenna body 11 through an axial aperture so that the central conductor 26 of the coaxial conductor 25 extends along the axis A. The central conductor 26 terminates in a loop having a rectangular configuration with major length L equal lit) the diameter of the coaxial conductor 25, and minor sides S effectively extensions of the outer conductor 31 of the coaxial. In a typical application, this loop employed for coupling electromagnetic energy from the coaxial conductor 25 to the cavity 14 is made up of 18-gauge copper wire, or may be formed from a finger which is an integral continuation of the outer conductor 25. The coaxial outer conductor is mounted for rotation in a cylindrical bore 27 of body 11 constituting a bearing surface. The outer conductor 25 is secured, as by brazing, to a gear 81 used to impart rotational motion to the outer conductor 25 and loop 30. A ring flange portion 28 constitutes a thrust bearing surface for the rotatable assembly. The flange portion 28 is retained in an annular groove in the body 11 by a retainer plate 29, the plate 29 and base of the annular groove providing the corresponding nonrotating bearing surfaces for the ring flange 28.

The loop 30 terminates at its inner end at a cap 37 which is spaced from the outer conductor 25 by an insulating collar 38. The cap 37 is also spaced from the inner conductor 26 by an air gap 39 providing capacitative coupling between the nonrotating center conductor 26 and the rotating loop 30.

The above-described arrangement is preferred for providing a simple loop-rotating mechanism. It is basically the well known rotating joint for coaxial lines of the type described and shown in E. G. Bowen, A Textbook of Radar, copyright 1954, Cambridge University Press, on page 185 and 186 and in FIG. 114. Other types of 1'0- tating coaxial joints may be used to fit the particular cavity and antenna requirements in accordance with established practice in the coaxial line art. The essential function of the rotating mechanism is to provide at least 90 of rotation of the cavity by the loop which extends into the cavity from the rear wall coaxially with the cavity axis.

The antenna body 11 includes integral pillars 33, 34, 35, and 36, two of which are shown in FIGURE 1 extending into the cavity 14. The pillars 33-36 extend into the cavity in a direction paral-lel to the longitudinal axis A at the cavity 14 and are all positioned equidistant from the axis and distributed symmetrically at 90 intervals around the axis A. Pillars 33 and 34 form a line parallel to the length of parallel slots 21 and 22, while pillars 35 and 36 are aligned normal to the length of slots 21 and 22.

The pillars 33-36 provide capacitative loading of the cavity 14 to reduce its size for any selected frequency in a manner well known in the art. The pillars 35 and 36, shown by FIG. 1, as well as pillar 33 of FIGURE 2, mount central tuning screws 40 which are used to adjust the resonant frequency of the cavity. The pillar 34, instead of a tuning screw, mounts a central probe 41 used as hereinafter described to couple energy into the cavity during the test phase of operation.

FIGURE 2 additionally shows the relative position of the apertured plate 15, and particularly slots 21 and 22, with respect to the input loop 30 and the loading pillars 33-36 in the cavity 14. The positioning of the plate 15 is established by an indexing pin in a Slot 51.

The dielectric window 20 forming the front face of the antenna is broken away for clarity, revealing the dumb bell-shaped slots 21 and 22. The slots 21 and 22 are equidistantly spaced from the axis with the length of the slot approximately equal to one-half of the wave-length of the intended frequency of operation. The circular end regions of the slots 21 and 22 provide additional perimeter length of the slot, allowing a lower frequency of operation than otherwise would be possible, fed by a cavity of the small diameter possible through the use of capacitative loading pillars 33-36.

The operation of this invention may be understood from FIGURES 3 and 4 illustrating the normal radiating or detecting mode of operation in FIGURE 3, and test operation in FIGURE 4. In both cases the cavity 14 is excited in the TM mode in which the magnetic or H field is oriented in the plane of the X and Y axis transversely to the Z axis of the cavity, and the electric or E field is oriented essentially along the Z axis, but varies in the plane of the X and Y axis, as shown in FIGURES 3b and 4b. The cavity 14 is excited by energy coupled in through the loop termination 30 of the coaxial lines in the second order mode with two maxima, as shown by the E field distribution arrows of FIGURES 3b and 4b. The voltage maxima points M and M in the cavity wall exist directly opposite the capacitative loading pillars 35 and 36, and the current distribution in the walls is indicated by the arrows appearing in FIGURES 3a and 4a. In FIGURE 3a, current flow is transverse to the length of slots 21 and 22, thereby excites the slots 21 and 22, and produces radiation therefrom or, if the antenna is operating as a receiving device, allows the coupling of properly polarized energy from the slots into the cavity to be detected by the loop 33. It is noted in FIGURE 3a that the direction of current flow through the slots 21 and 22 is the same whereby the slots are fed in phase, resulting in a cosine radiation pattern for the antenna in the transmitting mode. In the normal operation illustrated by FIGURES 3a and 3b, the probe is positioned along the X axis, which in this case coincides with the zero voltage plane of the Z axis (see FIGURE 3b). Therefore, the probe 41 is not excited by energy in the cavity during the normal operation.

FIGURE 4 shows the antenna with the orientation of the coupling loop changed by with respect to the antenna body. For convenience of showing the position of the probe within the cavity in FIGURE 4b, the body of the antenna, rather than the loop, is shown rotated 90 from the position of FIGURE 3. It should be noted that the shape of the cavity E field distribution is identical with the case of FIGURE 3b. However, the probe as shown in FIGURE 4b is now positioned at the point of voltage maximum M whereby energy is efliciently coupled between the cavity 14 and the probe 41. This is clearly illustrated by the current flow lines of FIGURE 4d. The cavity still excited in the TM mode has the same pattern as in the case of FIGURE 3a with current flowing from one point M coinciding with the position of a capacitative loading pillar 33 through the cavitys walls to the second maximum point M opposite the loading pillar 34. In both cases of FIGURES 3a and 4a, the direction of current flow is aligned with the direction of the loop 30. It should be noted in FIGURE 4a that there is no net current flow perpendicular to the length of the slots 21 and 22 as the current flowing across onehalf of the slots is opposite in direction to, and therefore cancels, that current flowing across the second half of the slot, whereby the slots are not excited significantly, and no radiation therefrom occurs. On the other hand, probe 41 capacitatively coupled to point M, of maximum voltage within the cavity is excited.

The operation of this invention may be further understood from FIGURE 5, showing in simplified form an illustrative system employing the antenna of FIGURES 1 and 2 as the receiving antenna and an antenna similar in construction to that of FIGURES 1 and 2 without the provision for rotation of the coupling loop and without any probe as the transmitting antenna. The system of FIGURE 5 is typically a radar-ranging system including a transmitter-receiver 60 having an indicator 61 calibrated to register the time delay between transmission of energy from a transmitting antenna until the detection of return in the receiving antenna as a measure of distance to a reflecting object. Transmitting and receiving antennas 62 and 63, respectively, are connected to the transmitterreceiver 60 by suitable transmission lines 64 and 65 which terminate in coupling loops 30. The transmission line 64 to the transmitting antenna 62 includes a coupler 76 for tapping off a small amount of the transmitted energy and introducing it through line 71 and a delay network 72 having a predetermined time delay. The output of the delay network 72 is connected to the probe ll of FIGURES 2, 3, and 4. The position of the loop 30 in the receiving antenna 63 is determined by a servo control system operated by a button 73 on the transmitterreceiver 60. The servo control system may include a motor 80 driving a gear system 81, as illustrated in FIG- URE 5, or simply a solenoid. The function of the servocontrol system is to position the loop of the receiving antenna normal to the length of the radiating slots. When the antenna is in its normal operating condition and responsive to the operation of the self-test button 73, the servo control rotates the loop 30 in the receiving antenna 63 90 to orient the loop for test purposes.

During the test operation, a portion of the energy directed to the transmitting antenna 62 and passing through the delay network 72 is coupled back through the internal probe 41, the cavity, and the coupling loop 30 to the receiver portion of the transmitter-receiver 60 where the known delay results in the needle of an indicator 60 moving to a position corresponding to the fixed delay. This position is identified by a test marker on the face of the indicator.

Whenever the operator wishes to determine whether the entire system is operating satisfactorily, he merely depresses the self-test button, and the next transmission to the transmitting antenna results in the needle moving rapidly to the test marker. Any other reading would indicate system malfunction.

The successful operation of the system of FIGURE 5 results from the presence of the antenna of FIGURES 1 and 2, designed for the normal efiicient transmission through the dual slots and for test operation in which the slots are not excited.

The two modes of operation are possible because of the simple, effective, noncontacting switching mechanism constituting an integral part of the energy-coupling system within the antenna.

Although for the purpose of describing the invention a particular embodiment thereof has been shown and described, obvious modifications will occur to a person skilled in the art, and I do not desire to be limited to the exact details shown and described.

I claim:

1. A slot antenna comprising:

a conductive body defining a cylindrical cavity,

one portion of said body defining an end wall of the cavity including a pair of substantially parallel slot apertures positioned on opposite sides of the longitudinal axis of the cylindrical cavity for coupling electromagnetic energy between the walls of the cavity and the external medium,

a conductive loop extending through the opposite end wall of said body into the cavity,

said conductive loop extending symmetrically with respect to the said cavity axis, and

means mounting said conductive loop for rotation about the said cavity axis to change the coupling efficiency between the slot apertures and said conductive loop.

2. A slot antenna comprising a body defining a cylindrical cavity, one end wall of the body defining said cavity including a pair of substantially parallel slots positioned symmetrically with respect to the longitudinal axis of the cavity,

loop means positioned in the opposite end Wall of said cavity for coupling electromagnetic energy between said loop means and the slots,

said loop means being substantially symmetrical about the longitudinal axis of the cavity, and

means mounting said loop means for selective positioning, either parallel to the length of said slots or at right angles thereto.

3. The combination in accordance with claim 2 wherein said body mounts electromagnetic energy-detecting means in the cavity oriented to be energized when said loop means is oriented at right angles with respect to said slots.

4. The combination in accordance with claim 3 including a plurality of capacitative loading members positioned symmetrically with respect to the longitudinal axis of the cavity and symmetrically with respect to said loop means when said loop means is in both the parallel and rightangle orientation with respect to said slots.

5. The combination in accordance with claim 4 wherein said capacitative loading means comprises four conductive pillars extending int-o the cavity parallel to the longitudinal axis thereof and symmetrically spaced therefrom.

6. The combination in accordance with claim 5 wherein one of said capacitative loading pillars mounts a probe for coupling energy between said cavity and the exterior of said body.

7. A conductive body defining a substantially cylindrical cavity, one portion of said body defining an end wall of the cavity including a pair of substantially parallel slot apertures positioned on opposite sides of the longitudinal axis of the cylindrical cavity for coupling electromagnetic energy between the walls of the cavity and the external medium,

a conductive loop extending through the opposite end wall of said body into the cavity,

said conductive loop extending symmetrically with respect to the said cavity axis,

said cavity being adapted for excitation in the TM mode such that its electric field is oriented essentially parallel to said axis.

3. A slot antenna as set forth in claim 7 wherein a pair of capacitive loading pillars are carried on said opposite wall, each of said pillars extending parallel to said longitudinal axis in close proximity to one of said slot apertures.

9. A slot antenna as set forth in claim 7 wherein said loop means is oriented substantially at right angles to said slots.

10. A slot antenna comprising a body defining a cylindrical cavity, one end wall of the body defining said cavity including a pair of substantially parallel slots positioned symmetrically with respect to the longitudinal axis of the cavity,

loop means positioned in the opposite end wall of said cavity for coupling electromagnetic energy between said loop means and the slots,

said loop means being substantially symmetrical about the longitudinal axis of the cavity,

said cavity being constructed and arranged for excitation in the TM mode in which the magnetic field is oriented in a plane essentially normal to said longitudinal axis and the electric field is oriented essentially parallel to said longitudinal axis.

11. A slot antenna as set forth in claim 10 wherein a pair of capacitive loading pillars are carried on said 0pposite wall, each of said pillars extending parallel to said longitudinal axis in close proximity to one of said slot apertures.

12. A slot antenna as set forth in claim 10 wherein said loop means is oriented substantially at right angles to said slots.

References Cited by the Examiner UNITED STATES PATENTS 2,460,286 1/1949 Hansen et al. 343-771 X 2,640,930 6/1953 Lundburg et al. 343770 X 2,972,147 2/ 1961 Wilkinson 343767 ELI LIEBERMAN, Acting Primary Examiner. 

1. A SLOT ANTENNA COMPRISING: A CONDUCTIVE BODY DEFINING A CYLINDRICAL CAVITY, ONE PORTION OF SAID BODY DEFINING AN END WALL OF THE CAVITY INCLUDING A PAIR OF SUBSTANTIALLY PARALLEL SLOT APERTURES POSITIONED ON OPPOSITE SIDES OF THE LONGITUDINAL AXIS OF THE CYLINDERICAL CAVITY FOR COUPLING ELECTROMAGNETIC ENERGY BETWEEN THE WALLS OF THE CAVITY AND THE EXTERNAL MEDIUM, A CONDUCTIVE LOOP EXTENDING THROUGH THE OPPOSITE END WALL OF SAID BODY INTO THE CAVITY, SAID CONDUCTIVE LOOP EXTENDING SYMMETRICALLY WITH RESPECT TO THE SAID CAVITY AXIS, AND 